84e9608596
Previously vec's len was updated only after full copy, making the method leak if T::clone panic!s. This commit makes `Vec::extend_from_within` (or, more accurately, it's `T: Clone` specialization) update vec's len on every iteration, fixing the issue. `T: Copy` specialization was not affected by the issue b/c it doesn't call user specified code (as, e.g. `T::clone`), and instead calls `ptr::copy_nonoverlapping`.